Dog retinas have been examined by thin-sectioning methods and transmission electron microscopy to define the variables in immersion fixation associated with good quality preservation under conditions of reproducibility. Incubation in a CSF medium enriched with glucose in a system exluding ambient air has resulted in remarkable integrity of the photoreceptor outer segments and surprisingly good preservation after longer incubation periods. Earlier work on the heterochromatin response of rod cell nuclei to ionic alterations has been repeated and reconfirmed. Additional studies using microperoxidase and ferritin as markers shows that these proteins mimic the distribution pattern of horseradish peroxidase in the extracellular compartment and in some synaptic and/or coated vesicles. Interreceptor junctions between rod spherules and cone pedicles have been found; this suggests that this species may possess the mechanism for electronic coupling between photoreceptor cells similar to that found in turtle, rabbit and monkey.